1. Field of the Invention
The present invention relates to a method of processing the surface of a semiconductor layer and, more particularly, a method of eliminating natural oxide film, which is formed on the surface of a matter such as a semiconductor wafer, and atoms and/or particles of other matters from the surface of said semiconductor wafer so as to clean the wafer surface. The present invention also relates to an apparatus for carrying out the above method.
2. Description of the Related Art
It is generally said that the natural oxide film (SiO.sub.2) formed on the surface of a silicon semiconductor wafer (which will be hereinafter referred to as the wafer) has 20 Angstroms. This natural oxide film is an extremely uneven insulator. When a wiring of aluminium (Al), for example, is formed on the natural oxide film upon manufacturing a semiconductor device, the insulating oxide film disturbs the connection between the wafer and the Al wiring. The semiconductor device thus produced cannot function as expected. In addition, the unevenness of the oxide film disturbs the close adhering of the Al metal relative to the wafer surface. A concrete example of this problem is shown in FIG. 3. A natural oxide film 52 formed on an Si wafer 51 interposes between an Al wiring or pickup electrode 53 and the wafer 51 to make wrong the contact between them. Reference numeral 54 in FIG. 3 denotes an insulating film between layers.
When the oxidized film is present on the base material upon forming a capacitor including a tantalum oxide (Ta.sub.2 O.sub.5) film thereon, the Ta.sub.2 O.sub.5 film is reduced to thereby degrade its function as dielectric.
As described above, it has been found that the natural oxide film adds extremely undesired influence to device characteristics. It has been energetically studied, therefore, in various research facilities how to eliminate the natural oxide film from the wafer surface. One of those methods which have been most widely employed these days is to immerse the wafer in a dilute HF solution. A passive film comprising 80% of Si atoms bonded to hydrogen (H) atoms and the remaining thereof bonded to fluorine (F) atoms has been found to be formed on the surface of the wafer which has been immersed in the dilute HF solution. It is therefore believed that the surface of the wafer thus immersed is inactive and effective to refrain the natural oxide film from being formed on the wafer surface.
When a thin film of Al or tungsten (W) is to be formed on the surface of Si layer, the above-mentioned passive film disturbs the formation of this thin film. The reason is that particularly F atoms are so strongly bonded to a part of Si atoms as not to be easily dissociated (bond dissociation energy=134 kcal/mol). These bonded parts are thus left between the thin film and the Si layer to thereby worsen the close adhering of the thin metal film relative to the Si layer. Or the peeling of the thin metal film off the Si layer is imagined at the worst. The quality of the thin metal film can be thus degraded. Further, when F atoms are attacked and eliminated from the Si layer by OH radicals or oxygen atoms, the above-mentioned bonded parts are oxidized at once as a layer in the horizontal direction. In other words, it has been found that these bonded parts are a key for igniting this so-called oxidization in the horizontal direction.
When Si atoms all over the wafer surface are bonded to H atoms, it is expected that a thin film having extremely fine characteristics can be formed, because Si atoms which have been bonded to H atoms are inactive for oxidization and because H atoms can be relatively easily replaced by other atoms when the thin film is formed. In order to bond Si atoms to H atoms, atoms and/or particles bonded to the surface of the wafer once treated in the dilute HF solution must be eliminated from the wafer surface to clean it.
It is well-known that the wafer surface is heated by infrared ray or irradiated with ray from an Xe lamp in order to eliminate the H/F passive film from the wafer surface. It is also well-known that the wafer surface is washed by pure water after its dilute HF treatment.
In the case of these infrared ray heating and Xe lamp irradiating methods, however, a temperature of about 400.degree. C. is needed for the elimination of H atoms and a temperature of about 900.degree. C. is needed for the elimination of F atoms. The wafer is therefore subjected to thermal degradation and its quality is thus worsened. In the case of the pure water washing method, there is no fear that the wafer is subjected to thermal degradation but contaminants in the pure water cause current to be leaked and the insulating capacity of the wafer to be worsened. In addition, it is difficult to remove the contaminants from the pure water.